Thermopiles have been used to evaluate remotely the temperature of zones in several applications as different as ear thermometer (Kenneth et al, U.S. Pat. No. 4,722,612), interior of microwave oven (Bu, U.S. Pat. No. 5,589,094; Lee, U.S. Pat. No. 6,013,907), hair dryer (Van Der Wal et al, WO 99/01726), heating system burner (Carignan, U.S. Pat. No. 4,717,333), and motion detection (Beerwerth et al, U.S. Pat. No. 6,203,194 B1). In the latter case, motion detection is obtained by using a multiple lens technology in order to generate several variations on a sensor array each time the target leaves a zone covered by one lens to enter another one.
Active sensor devices used for presence or motion detection generally rely on the principle that the device comprises an emitter that emits radiation (typically electromagnetic radiation such as light or microwave) in the direction of an area (for example on the ground or there above) or a solid angle in space to be surveyed (herein referred to as a surveillance area) and a radiation detector for detecting a portion of the radiation re-emerging from the surveillance area for example by its reflectivity. The amount of re-emerging radiation varies, when the reflectivity in the direction of the radiation detector is altered. This may occur for example as a consequence of an object entering the surveillance area.
Active sensors generally, and also in applications for automatic doors, have the following disadvantages. Emitting radiation causes an according supplementary power consumption required to maintain the emission of the radiation. The emitted radiation may be detrimental to the well-being or health of humans. When visible, the emitted radiation may also be conspicuous when it is not desirable that presence monitoring is being performed. And the detected amount of radiation reemerging from the surveillance area may be altered by objects other than the target objects to be detected, e.g. human bodies, which may lead to “false alarms”.
Passive sensor devices for presence detection do not emit radiation to perform their detection, but rather detect radiation emitted by the target object to be detected. Pyro-electric sensors are well known in applications around automatic doors, but they are being used for movement detection. Pyro-electric sensors are only providing temperature variation measurements, while not being suitable for steady-state temperature measurements.